This disclosure relates to multilayered articles with low optical retardation, and in particular to multilayered article compositions, methods of manufacture, and uses thereof.
Optical sheets/films used for liquid crystal display applications can be made with a variety of transparent thermoplastic materials. The desired properties of the thermoplastic materials for these applications include high transparency, good color, high clarity, high glass transition temperature, toughness, ductility, dimension stability, chemical resistance, melt processability, etc. Polycarbonate satisfies many of the above requirements. However, optical sheets/film or disks made from polycarbonate (i.e. Bisphenol-A (BPA) polycarbonate homopolymer (BPA-PC)) possess relatively high optical retardation if they are formed using a melt extrusion or a molding process that involves cooling of the polycarbonate resin from melt to solid under pressure or stress. Examples of such extrusion or molding processes include calendering sheet/film extrusion, profile extrusion, embossing, injection molding and compression molding process, etc., which are used for the purpose of forming the shape and/or the surface finish or texture of the articles being molded.
High optical retardation level of an optical sheet can be associated with high non-uniformity in distribution of optical retardation across a large area due to a non-uniform pressure applied to the optical sheets during their melt processing step. As a result, the non-uniformly distributed optical retardation in the sheets causes visually undesirable color band issues (“Rainbow Mura” or “Rainbow appearance”) for the applications related to liquid-crystal displays (LCD). Optical films with light management textures (such as micro-lens, prismatic, or engineered random textures) and non-uniformly distributed optical retardation are not ideal for use in a top film layer in the display film stack used in Back Light Modules (BLM) of LCD displays because of the “Rainbow Mura” issue. In addition, for reflective polarizer sheets used in some LCD displays, a high optical retardation level in the top light diffusing layer can cause light depolarization, hence leading to a loss of brightness enhancement effect of the reflective polarizer sheet.
Previous attempts to make an optical thermoplastic sheet, film or disk with low retardation include the following approaches: 1) using a material with inherently low birefrigence as a substitute for the material with inherently high birefrigence; 2) reducing the residual mechanical stress in the formed article (sheets/films/disks) during the extrusion or molding process, or by secondary annealing processes.
In accordance with the first approach, examplary materials with inherent low birefrigence include cyclic olefin copolymer (COC), specialty polyester copolymers such as OKP4 from Osaka Gas Chemical, and poly(methyl methacrylate) (PMMA). However, these materials can be expensive and/or exhibit poor impact resistance (e.g., PMMA).
In accordance with the second approach, in an effort to reduce the residual stress in an extruded optical sheet/film, a reduction in the mechanical loads (pressure or web tension) is applied to the polymer melt while the melt solidifies in the sheet extrusion processes casting the molten web onto a single chill roll without nipping, or using a compliant roll as one of the nip rollers for the melt calendering extrusion process. These approaches can enable low optical retardation of the resulting sheet through reduced residual stress, but can also limit the ability to impart functional surface features (e.g., micro-lenses, prismatic features, or matte patterns) to the film or sheet for light management purposes due to lack of sufficient nip pressure in a melt calendering process. If the compliant nip roll is a roll with rubber surface, this approach also suffers a low extrusion line throughput due to the inefficient cooling of the polymer melt at the rubber roll surface which is not a good heat conductor.
Alternatively, a substrate film can be coated with a cross-linkable liquid coating (e.g., a crosslinkable acrylic coating liquid) at one of its surfaces. A light management surface texture is then introduced to the liquid coating surface via micro-replication process followed by solidification of the texture pattern by a UV radiation process. The micro-replication process using liquid monomer coating typically does not cause significant increase in optical retardation of the finished film. However, this method is more costly than a direct calendering extrusion or injection-molding process. In addition, the multilayer article is not 100 percent thermoplastic polymer and cannot be easily recycled.
Therefore, there is a need for thermoplastic optical films and sheets with a light management surface texture, low optical retardation, and good mechanical properties that are produced through a cost effective method.